Expert opinion on antimicrobial therapies: is there enough scientific evidence to state that targeted therapies outperform non-targeted ones?

INTRODUCTION: Different active and passive strategies have been developed to fight against pathogenic bacteria. Those actions are undertaken to reduce the bacterial burden while minimizing the possibilities to develop not only antimicrobial resistance but also antimicrobial side-effects such as allergic or hypersensitivity reactions. AREAS COVERED: We have reviewed preclinical results that evidence that targeted antimicrobial therapies outperform non-targeted ones. Active selective targeting against pathogenic bacteria has been achieved through the functionalization of antimicrobials, either alone or encapsulated within micro- or nanocarriers, with various recognition moieties. These moieties include peptides, aptamers, antibodies, carbohydrates, extracellular vesicles, cell membranes, infective agents, and other affinity ligands with specific bacterial tropism. Those selective ligands increase retention and enhance effectiveness reducing the side-effects and the required dose to exert the antimicrobial action at the site of infection. EXPERT OPINION: When using targeted antimicrobial therapies not only reduced side-effects are observed, but also, compared to the administration of equivalent doses of the non-targeted drugs, a superior efficacy has been demonstrated against planktonic, sessile, and intracellular pathogenic bacterial persisters. The translation of those targeted therapies to subsequent phases of clinical development still requires the demonstration of a reduction in the probabilities for the pathogen to develop resistance when using targeted approaches.

Antimicrobial Photodynamic Therapy Using Encapsulated Protoporphyrin IX for the Treatment of Bacterial Pathogens.

Herein, we report on the antimicrobial photodynamic effect of polymeric nanoparticles containing the endogenous photosensitizer protoporphyrin IX. Compared to equivalent doses of the free photosensitizer, we demonstrated that the photodynamic antimicrobial efficacy of PLGA (polylactic-co-glycolic acid) nanoparticles containing protoporphyrin IX (PpIX) against pathogenic Staphylococcus aureus (S. aureus) is preserved after encapsulation, while photobleaching is reduced. In addition, compared to equivalent doses of the free porphyrin, we show that a reduction in the cytotoxicity in mammalian cell cultures is observed when encapsulated. Therefore, the encapsulation of protoporphyrin IX reduces its photodegradation, while the released photosensitizer maintains its ability to generate reactive oxygen species upon light irradiation. The polymeric nanoencapsulation promotes aqueous solubility for the hydrophobic PpIX, improves its photostability and reduces the cytotoxicity, while providing an extended release of this endogenous photosensitizer.

Antimicrobial Wound Dressings against Fluorescent and Methicillin-Sensitive Intracellular Pathogenic Bacteria.

There is limited evidence indicating that drug-eluting dressings are clinically more effective than simple conventional dressings. To shed light on this concern, we have performed evidence-based research to evaluate the antimicrobial action of thymol (THY)-loaded antimicrobial dressings having antibiofilm forming ability, able to eradicate intracellular and extracellular pathogenic bacteria. We have used four different Staphylococcus aureus strains, including the ATCC 25923 strain, the Newman strain (methicillin-sensitive strain, MSSA) expressing the coral green fluorescent protein from the vector pCN47, and two clinical reference strains, Newman-(MSSA) and USA300-(methicillin-resistant strain), as traceable models of pathogenic bacteria commonly infecting skin and soft tissues. Compared to non-loaded dressings, THY-loaded polycaprolactone-based electrospun dressings were also able to eliminate pathogenic bacteria in coculture models based on infected murine macrophages. In addition, by using confocal microscopy and the conventional microdilution plating method, we corroborated the successful ability of THY in preventing also biofilm formation. Herein, we demonstrated that the use of wound dressings loaded with the natural monoterpenoid phenol derivative THY are able to eliminate biofilm formation and intracellular methicillin-sensitive S aureus more efficiently than with their corresponding THY-free counterparts.

Antibacterial Effect of Thymol Loaded SBA-15 Nanorods Incorporated in PCL Electrospun Fibers.

For the effective management of infected chronic wounds, the incorporation of antimicrobial drugs into wound dressings can increase their local availability at the infection site. Mesoporous silicon dioxide SBA-15 is an excellent drug carrier with tunable drug release kinetics. In this work, synthesized SBA-15 loaded with the natural antimicrobial compound thymol (THY) was incorporated into polycaprolactone (PCL) electrospun nanofibers to obtain an advanced wound dressing. Rod-shaped particles with internal parallel channels oriented along the longitudinal axis (diameter: 138 ± 30 nm, length: 563 ± 100 nm) were loaded with 70.8 wt.% of THY. Fiber mats were prepared using these particles as nanofillers within polycaprolactone (PCL) electrospun fibers. The resulting mats contained 5.6 wt.% of THY and more than half of this loading was released in the first 7 h. This release would prevent an initial bacterial colonization and also inhibit or eliminate bacterial growth as in vitro shown against Staphylococcus aureus ATCC 25923. Minimal inhibitory concentration (MIC: 0.07 mg/mL) and minimal bactericidal concentration (MBC: 0.11 mg/mL) of released THY were lower than the amount of free THY required, demonstrating the benefit of drug encapsulation for a more efficient bactericidal capacity due to the direct contact between mats and bacteria.

Efficiency of Antimicrobial Electrospun Thymol-Loaded Polycaprolactone Mats In Vivo.

Due to the prevalence of antimicrobial resistant pathogens, natural products with long-term antimicrobial activities are considered as potential alternatives. In this work, polycaprolactone (PCL) electrospun fibers with mean diameters around 299 nm and loaded with 14.92 ± 1.31% w/w thymol (THY) were synthesized. The mats had appropriate elongation at break (74.4 ± 9.5%) and tensile strength (3.0 ± 0.5 MPa) to be potentially used as wound dressing materials. In vivo studies were performed using eight to ten week-old male SKH1 hairless mice. The infection progression was evaluated through a semiquantitative method and quantitative polymerase chain reaction. The analyses of post-mortem samples indicated that THY-loaded PCL fibers acted as inhibitors of Staphylococcus aureus ATCC 25923 strain growth being as efficient as chlorhexidine (CLXD). Histopathological and immunohistochemical studies showed that the PCL-THY-treated wounds were almost free of an inflammatory reaction. Therefore, wound dressings containing natural compounds can prevent infection and promote wound healing and prompt regeneration.

Antimicrobial Electrospun Polycaprolactone-Based Wound Dressings: An In Vitro Study About the Importance of the Direct Contact to Elicit Bactericidal Activity.

Objective: To prepare efficient antibacterial carvacrol (CAR) and thymol (THY)-loaded electrospun polycaprolactone (PCL)-based wound dressings. Approach: Using electrospinning we were able to prepare wound dressings with antimicrobial action thanks to their large surface per volume ratio, which allows their loading with therapeutic amounts of active principles. By nuclear magnetic resonance we demonstrated that the antimicrobial compounds are donors of hydrogen bonds to the ester functional group in PCL, which acts as acceptor and that intermolecular interaction is responsible for the high drug loading achieved. Results: Those mats loaded with CAR and THY without the use of solubilizing agents were able to completely eradicate both Gram-positive (Staphylococcus aureus ATCC 25923) and Gram-negative (Escherichia coli S17 strain) bacteria at doses inferior to the ones needed when using the free nonsupported compounds. A superior antimicrobial action was observed for THY and CAR against Gram-negative bacteria than against Gram-positive bacteria, despite the higher hydrophilicity of the outer layer of Gram-negative bacteria. Innovation: We demonstrate that a direct contact between the bacteria and the dressing is required to elicit antimicrobial action. We also evaluated drug loadings by gas chromatography coupled with mass spectrometry and nuclear magnetic resonance validating a new analytical approach. Finally we were able to visualize the pathogenic bacteria on the dressings by confocal microscopy. Conclusion: The interaction between the PCL-based mat and the pathogenic bacteria is a key issue to achieve complete pathogen eradication. Under no-contact conditions, released CAR or THY from the electrospun mats did not exert any antimicrobial action at the doses tested.